Quick question.....I have a 90 gallon that is on my DIY stand. The stand itself is leveled with shims, however when I put the tank on the stand, which has a piece of plywood, it is slightly unlevel. There is a tiny tiny wobble. The aquarium is trimmed (black), so would it be possible and safe if I took a piece of say 1/2" foam of somet type(suggestions), and cut out a rectangle that would have the tank trim rest on the stryfoam and leave the glass bottom with no foam. I do worry, because the tank is trimmed, that the foam would cause pressure on the bottom glass, thus I thought of cutting to fit the trim only. Any thoughts?

I use 1/2" thick styrofoam from Home Depot under my tanks.
Many of them have just a shell of Styrofoam, perhaps 2" wide, just under the rim of the tank. The tanks like this are glass, with the weight intended to be just on the rim. My acrylic tanks, where the weight is supposed to be over all the surface have full sheets of styrofoam. This works well to even out the very minor imperfections such as the grain of the wood. It is never intended to solve a wobble. The stand and tank are stable, plumb, level and square before I add the styrofoam.

If the wobble is just the tank, and the problem is perhaps 1/16", when the tank is empty, but it settles and is stable when it is full this might not be a problem. But for a bigger gap, say 1/8" then fix the problem. Get a new piece of wood, or fix the piece you have.
You can start with coarse sanding sheets to knock down the worst area, but do not over work it. Leave it up a bit and shift to finer sanding sheets. If you keep working with the most aggressive sanding sheets you do indeed run the risk of over sanding, and creating a depression where there was a hill.
Stop often to see how it is going.
A 4' level is a good idea. Keep sliding it over the area, and shifting it around in all directions.

I use 1/2" thick styrofoam from Home Depot under my tanks.
Many of them have just a shell of Styrofoam, perhaps 2" wide, just under the rim of the tank. The tanks like this are glass, with the weight intended to be just on the rim. My acrylic tanks, where the weight is supposed to be over all the surface have full sheets of styrofoam. This works well to even out the very minor imperfections such as the grain of the wood. It is never intended to solve a wobble. The stand and tank are stable, plumb, level and square before I add the styrofoam.

If the wobble is just the tank, and the problem is perhaps 1/16", when the tank is empty, but it settles and is stable when it is full this might not be a problem. But for a bigger gap, say 1/8" then fix the problem. Get a new piece of wood, or fix the piece you have.
You can start with coarse sanding sheets to knock down the worst area, but do not over work it. Leave it up a bit and shift to finer sanding sheets. If you keep working with the most aggressive sanding sheets you do indeed run the risk of over sanding, and creating a depression where there was a hill.
Stop often to see how it is going.
A 4' level is a good idea. Keep sliding it over the area, and shifting it around in all directions.

Thanks for then reply Diana!

Just so I am clear you have rimmed tanks like the one I described and you use the foam only on the edges? I have heard so many say use it, don't use it. Have you run into any issues with it? Did you just cut your piece and measure the width of the rim and have the foam sit just on the rim and not underneath the bottom glass?

So I can be sure what you mean: the tank is sitting 'level' but you can slide a piece of paper between the stand and the tank in a few places and this causes the tank to wobble?
I would use styrofoam. I like the extra dense pink or purple stuff and not the white styrofoam that leave little white pills everywhere. Technically I think it is 'polystyrene' and not 'stryofoam'.
To hide the unsightly pink pad I pick up some pieces of baseboard trim of an appropriate size and border the styrofoam. When I feel like being fancy I let the styrofoam be a slightly larger size than the tank (1" all around) and I choose trim that is tall enough (wide enough, whatever) to cover the styrofoam AND the bottom plastic ring of the tank. This makes a space around the outside of the tank sort of like a moat. I tape the seams with duct tape and then fill it with whatever substrate is in the tank. This gives the illusion that the substrate in the tank comes right out of the tank to the edge of the trim. It looks tidy and clean and fully hides the plastic rim and foam.

Edit: I have done this with a plastic ring 50 gallon tank and I used a single solid piece of styrofoam under the entire tank. If you are concerned about the glass hitting foam, you can test the compressive strength of the foam by putting something the same width as the plastic trim on top of a piece of styrofoam and standing on it. Use a short piece and you should be able to approximate the PSI that the tank will be applying.

So I can be sure what you mean: the tank is sitting 'level' but you can slide a piece of paper between the stand and the tank in a few places and this causes the tank to wobble?
I would use styrofoam. I like the extra dense pink or purple stuff and not the white styrofoam that leave little white pills everywhere. Technically I think it is 'polystyrene' and not 'stryofoam'.
To hide the unsightly pink pad I pick up some pieces of baseboard trim of an appropriate size and border the styrofoam. When I feel like being fancy I let the styrofoam be a slightly larger size than the tank (1" all around) and I choose trim that is tall enough (wide enough, whatever) to cover the styrofoam AND the bottom plastic ring of the tank. This makes a space around the outside of the tank sort of like a moat. I tape the seams with duct tape and then fill it with whatever substrate is in the tank. This gives the illusion that the substrate in the tank comes right out of the tank to the edge of the trim. It looks tidy and clean and fully hides the plastic rim and foam.

Thanks for the reply!

It is along the back of the stand about 35 inches from the right side to left....as I go to the left side the gap becomes smaller, it is about 1/16 off....when the tank does not dip to that corner then yes it is level. For a better reference in that corner I can slid a quarter under it. I am leaning towards sanding but I worry I will make it worse.....I could sand it and if it is worse just rip out that piece of plywood and buy another piece (check for it being level at the store).

I personally use 3/4 inch pink rigid foam under my 220g for the same reason . It will conform to the tank and stand as needed to keep stress off of the tank .

Hope this helps

Thanks for the reply!

My counter to that is....does the foam really alleviate the pressure points (where the tank is actually on the stand top flat) or is it just hiding it by putting more pressure on the areas that were touching which is now compressing the foam harder. Doesn't it seem like the foam would just keep the pressure points on where the top meets the trim? I just have some skepticism about it that's all

Doesn't it seem like the foam would just keep the pressure points on where the top meets the trim?

With a large gap this is true.
With a small gap the points of high pressure will squash into the styrofoam until the points with low pressure start to support the load. The net result being that every square inch of tank bottom with have the same psi of pressure as any other square inch.

As long as the gap between tank and stand doesn't exceed the depth of the 'squish' you should be alright. I only use styrofoam as a solution to small gaps. Your 'quarter dollar' gap is about as large as I would use styrofoam on. I would recommend using that level and seeing if you can bring it a little flatter before you put down the foam. I usually measure gaps in 'sheets of paper' and not in 'coins'.

If you are handy with math, you can figure out how much PSI (pounds per square inch) the rim of that tank supports, then with a ratio you can make a small block that will have equal PSI when your body weight is standing on it. Then you can test a piece of the foam you have chosen to see how much 'squish' it has. if the squish is less than the distance between stand and glass (height the plastic ring raises the tank) you could use a full piece without worrying about that age old concern of the glass on a rimmed tank touching the styrofoam.

Just a thought here....but since my tank sits on a rim, inm theory, the only part of the "top" that needs to be level is where the trim meets the stand, right? Am I correct in this that I do not have to sand/level any area that is not in contact with the trim?

I am also thinking of taking jigsaw and cutting 2 square holes in the stand top, to make it a modified open top with the plywood only situated where the trim meets the stand

My personal thought on a tank being level is ; as close as humanly possible. Reason being water is heavy and glass is fragile . Causing any extra stress on the tank from not being level is asking for potential problems. That being said will the tank fail if its an inch off level ? Probably not .. but if you have the ability to make it level do your best .....just my $.02....

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